Chemical vapor deposition (CVD) is a well known process for the fabrication of semiconductor integrated circuits and other layered structures formed on a substrate. In CVD, the semiconductor wafer or other substrate is exposed to a precursor gas at a reduced pressure inside a vacuum chamber. The precursor gas reacts at the surface of the wafer and deposits a component on the wafer. For example, silane (SiH.sub.4) is often used as the precursor gas for depositing silicon, and TEOS (tetraethylorthosilicate) is often used for silicon dioxide. There are two principal ways of driving the reaction. If the wafer is heated to a sufficiently high temperature, the reaction is thermally activated. However, for many applications, the temperature necessary for an efficient rate of thermal activation is considered to be too high. In an alternative method, called plasma-enhanced CVD or PECVD, electrical means are used to excite the precursor gas into a plasma. The plasma creates ions and/or radicals of the precursor gas and its components, and these much more readily react. Thereby, the temperature of the wafer can be held fairly low.
An example of a PECVD reaction chamber is described by Zhao et al. in U.S. Pat. No. 5,558,717, incorporated herein by reference. This type of CVD reactor is available from Applied Materials, Inc. of Santa Clara, Calif. under the name DxZ chamber. The CVD reactor of the patent is illustrated in the cross-sectional view of FIG. 1. An unillustrated wafer is supported during processing on a pedestal 10, which can be lowered for the loading and unloading of the wafer into and out of the vacuum chamber through a slit-valve opening 12 in a lower chamber body 14 and a ceramic ring 16 inside the lower chamber body 14.
During deposition, a precursor gas 18 flows through a center-feed distribution system overlying the wafer and through a large number (several thousands) of jet holes 20 in a faceplate 22 composed of a conductive metal, such as aluminum. The gas distribution system is described by Schneider et al. in U.S. patent application Ser. No. 08/734,015, filed Oct. 18, 1996 and entitled INDUCTIVELY COUPLED PARALLEL-PLATE PLASMA REACTOR WITH A CONICAL DOME. The front portion of the faceplate 22 containing the jet holes 20 is called a showerhead 24. As illustrated, during processing the showerhead 24 is closely opposed to the wafer, and its perforated area extends substantially coextensively with the area of the wafer. The processing gas flows through the showerhead holes 20, over the wafer, and then generally radially outwardly to an annular pumping channel 26, generally surrounding the upper edge of the pedestal 10 during processing. The spent gas is exhausted through a restriction 28 in the pumping channel 26 to an exhaust manifold 29. A valve 30 gates the exhaust to an exhaust vent 32 which is pumped by an unillustrated vacuum pumping system.
The faceplate 22 and associated parts are held in a lid frame 34, which is sealed to the lower chamber body 14 by O-rings 36. The lid frame 34 pivots about an unillustrated horizontal hinge and thus can be raised away from the lower chamber body 14 in order to allow technicians to service the interior of the chamber. As shown, the pumping channel 26 is formed between the lid frame 34, the lower chamber body 14, and a second ceramic ring 38 supported through the first ceramic ring 16 on the chamber body 14, but the pumping channel 26 mainly extends into the lid.
The illustrated reactor is intended to be used as a plasma reactor. The pedestal 10 is typically grounded while a cover 40 both electrically and mechanically fixed to the faceplate 22 is connected to an RF power supply 42. Thus, a processing space 44 is surrounded by RF-driven electrodes consisting of the showerhead 24 and the pedestal 10. Sufficient RF power is applied so that the process gas in the processing space 44 between the showerhead 24 and the pedestal 10 is excited into a plasma to activate the CVD reaction on the surface of the wafer. Thereby, the reaction can be carried out at relatively low temperatures with little effect on the thermal budget of the integrated circuit being formed.
The lower chamber body 14 is usually made of a metal such as aluminum and, for safety reasons, is electrically grounded. An annular isolator 46 electrically isolates the RF-driven faceplate 24 from the lid frame 34 and the lower chamber body 14, to which it is electrically connected. The isolator 46 is formed either of a ceramic material, such as alumina or of a sturdy plastic, such as Teflon, both of which provide good electrical isolation.
The chamber of FIG. 1 was designed for 200 mm (8-inch) wafers. Scaling these chambers up for 300 mm (12-inch) wafers presents further problems as well as an opportunity to improve the basic design.
In the reactor of FIG. 1, the temperatures of the faceplate 22 and its showerhead 24 are not tightly controlled. The pedestal 10 is actively heated by resistive coil, but the faceplate 22 is not actively heated or cooled. The temperature of the showerhead 24 is estimated to be about 200.degree. C. because of collisional heating from the plasma and radiative heating from the pedestal 10. Although the temperature tends to equilibrate between the pedestal 10 and the showerhead 24, some of the showerhead heat is sunk through the outer and upper portions of the faceplate 22 to the lid frame 34 and eventually to the lower chamber body 14, as well as to other peripheral attached parts.
Such temperatures are not extreme, but they still cause reliability and lifetime problems in the O-rings sealing the faceplate 22, the lid frame 34, chamber body 14 and other parts. It is thus desirable to reduce the temperature at the back of the faceplate 22.
The heat sinking at the circumference of the showerhead causes at least two related problems. Heat production is substantially uniform over the area of the showerhead, and heat flows to the cooler circumferential area over a path having a substantially uniform thermal conduction. As a result, the center of the showerhead 24 has a higher temperature than the showerhead portions nearer the circumference. The radial non-uniformity in temperature affects the uniformity of the deposition rate and also introduces thermal stresses into the showerhead 24. The thermal stresses cause the showerhead 24 to bow, and the resultant variable gap size of the processing space introduces a non-uniform plasma, yet another source of non-uniformity in deposition. These temperature non-uniformities significantly worsen for the larger wafer sizes.